1. Field of the Invention
The invention relates to methods for manufacturing a Complementary Metal Oxide Semiconductor (CMOS) image sensor.
2. Description of the Related Art
In general, an image sensor is a semiconductor device capable of converting an optical image into an electrical signal. Image sensors can generally be classified into Charge Coupled Device (CCD) image sensors and Complementary Metal Oxide Semiconductor (CMOS) image sensors.
In CCD image sensors, a plurality of Metal Oxide Silicon (MOS) capacitors are positioned adjacent to each other. Charge carriers can be stored in each capacitor and transferred to its adjacent neighbors.
CMOS image sensors employ a switching way for forming MOS transistors corresponding to a quantity of unit pixels in a semiconductor substrate. The unit pixels use a CMOS technology with a control circuit and a signal processing circuit as peripheral circuits. CMOS image sensors can therefore sequentially detect an output of each unit pixel by the MOS transistors. In greater detail, a CMOS image sensor includes a photodiode and a Metal Oxide Semiconductor (MOS) transistor within each unit pixel and thus, sequentially detecting an electrical signal of each unit pixel is possible.
CMOS image sensors use CMOS fabrication technology and thus, exhibit low power consumption and are relatively simple to fabricate due to fewer required photo processes. CMOS image sensor are also relatively simple to miniaturize because a control circuit, a signal processing circuit, and an analog-to-digital conversion circuit can be integrated into a CMOS image sensor chip. Accordingly, CMOS image sensors are widely used in diverse applications, such as digital still cameras and digital video cameras.
In the CMOS image sensor, color filters are arranged over a light sensing portion for receiving light from the exterior and generating and accumulating optical charges. Color Filter Arrays (CFAs) generally include three colors: red, green, and blue; or yellow, magenta, and cyan.
An image sensor includes a light sensing portion for sensing light and a logic circuit portion for converting the sensed light into an electrical data signal. In order to enhance photosensitivity, attempts have been made to increase a fill factor between an occupation area of the light sensing portion and a total area of the image sensor. However, there is a limit to effectiveness of these attempts because the logic circuit portion cannot be fundamentally uninstalled. Accordingly, in order to increase photosensitivity, a focusing technology has been developed that changes a path of light incident on regions other than the light sensing portion and focuses the incident light on the light sensing portion. This focusing is accomplished by forming a micro lens over a color filter.
However, where forming a micro lens over a color filter, a fill factor of an occupation area of a light sensing portion gets smaller as a pixel size gets smaller. Thus, due to a shortage of a quantity of incident light, photoelectric conversion charges consequently reduce in number and thus photosensitivity reduces. As a result, the image sensor is deteriorated in quality. Accordingly, in order to compensate for a poor sensitivity caused by a reduction of the fill factor of the occupation area of the light sensing portion, attempts have been made at overcoming the reduction of the pixel size by forming a focusing micro lens more efficiently and enhancing sensitivity.
FIGS. 1A to 1E are process cross-sectional diagrams of a prior art method for manufacturing a CMOS image sensor.
As disclosed in FIG. 1A, an insulating film 101 is formed on a semiconductor substrate 100. Next, a metal pad 102 for each signal line is formed over the insulating film 101. Then, a protection film 103 is formed on the interlayer insulating film 101 and the metal pad 102. The protection film 103 is formed of oxide or nitride.
As disclosed in FIG. 1B, a photosensitive film 104 is then coated on the protection film 103. Next, the photosensitive film 104 is selectively patterned to expose an upper portion of the metal pad 102 in an exposure and development process. A pad opening portion 105 is formed at the metal pad 102 by selectively etching the protection film 103 using the patterned photosensitive film 104 as a mask.
As disclosed in FIG. 1C, the photosensitive film 104 is then removed. Next, the semiconductor substrate 100 is processed by wet cleaning. Then, a first planarization layer 106 is deposited on the protection film 103. A blue color filter layer 107, a green color filter layer 108, and a red color filter layer 109 are subsequently formed on the first planarization layer 106.
As disclosed in FIG. 1D, a second planarization layer 111 is then formed over a portion of the first planarization layer 106 and the color filter layers 107, 108, and 109.
As disclosed in FIG. 1E, micro lenses 112 are then formed corresponding to the respective color filter layers 107, 108, and 109 on the second planarization layer 111. A probe test is next performed for each metal pad 102 of the above-constructed CMOS image sensor to check a contact resistance. Then, if the contact resistance is not in an abnormal condition, the metal pad electrically connects with an external driving circuit.
One disadvantage of the prior art method for manufacturing a CMOS image sensor disclosed in FIGS. 1A-1E is that the resolution of the micro lens 112 is reduced due to the second planarization layer 111 being formed using an i-line or g-line Middle UltraViolet (MUV) photoresist. A stripe patterned defect is caused by a CD difference because an MUV pattern with a bad uniformity reflows and increases in size resulting in increasing the size of the micro lens.